Mitel 5320 Reference Manual page 122

To configure Call Coverage using your mobile device, follow these steps:
1. Log into the mobile webadmin.
2. Tap Call Coverage.
3. Edit the Number of Rings Before Sending Call to Call Coverage field.
4. Edit the Call Coverage Type field.
5. Edit the Call Coverage Destination field.
If you do not enable this field, all calls go to treatment.
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Mitel IP Phone Reference Guide
Mobile Webadmin